1. Field of the Invention
This invention relates to a logic circuit and more particularly to a three-level input logic circuit for receiving at its input terminal and processing a logic input having three different levels.
2. Description of the Prior Art
In an ordinary digital circuit, logic operation is carried out by using the combinations of binary quantities (e.g. high level and low level). Accordingly, if the external input applied to an input terminal is of analog quantity, the level of the input is forced to be processed by an analog-digital converter which converts an analog quantity to a binary quantity. This means that N input terminals can receive only 2.sup.N different combinations of the levels. A variety of such circuits as to provide multiple levels have already been proposed in the field of the art under consideration. For example, the specification of the U.S. Pat. No. 3,939,676 to Minoru Fujita, discloses a multi-level voltage circuit for liquid cell display device. In all the prior art propositions, however, only two levels are derived from an individual input. This also leads to the fact that N input terminals can only receive 2.sup.N different combinations of levels. Consequently, all the circuits hitherto proposed have a drawback from the standpoint of reducing the number of electrodes or contacts in a semiconductor integrated circuit.
In case the external input is derived from, for example, a mechanical switch, three states (i.e. a power source level, an open level and a ground level) can be easily realized. It is therefore required to provide an input circuit for conducting the three levels into a logic system.